1. Field of the Invention
The present invention is directed to a computed tomography apparatus of the type wherein an x-ray fan beam is caused to be rotated around an examination subject to irradiate the subject from a series of different angular positions.
2. Description of the Prior Art
In computed tomography systems of the type generally described above, the x-ray source which emits the fan beam, and the x-ray detector, are rotated around a rotational center on a rotating frame. The rotational center and the center of gravity of the focus of the focal spot of the x-ray tube should lie on a straight line for all angular positions. If, due to mechanical wear or play, the rotational center changes in position, the system is out of adjustment.
In a system which is out of adjustment, the fan beam geometry no longer corresponds to the theoretical geometry which is used in the image computation, and thus results in a loss of image sharpness and a degradation of the modulation transfer function (MTF).
Rotational bearings having extremely tight tolerances are consequently required in a computed tomography apparatus. The position of the fan beam must be readjusted in the event of a change in the position of the rotational center of the bearing. Moreover, the bearing must be replaced if it exhibits a constant, drift-like variation in the position of the rotational center, since the constant readjustment which is necessary under such conditions has a highly limiting effect on the use of the system.
A computed tomography system is described Patent Abstracts of Japan, P-1285, Dec. 6, 1991, Vol. 15/No. 483 having an x-ray source which irradiates a specimen, with the specimen being rotated around a rotational center. The x-ray source, the rotational center of the specimen, and the center of the x-ray detector in this known apparatus always lie on the same connecting straight line. This computed tomography apparatus, however, is not suitable for examination of patients for medical purposes, since medical computed tomography systems do not rotate the patient.
A computed tomography apparatus is disclosed in United Kingdom application 15 84 954 having means for deflecting the focus of the x-ray tube. The focus deflection, however, is not for the purpose of compensating undesired dislocations of the rotational center. Instead, the focus displacement in this known system ensues for the purpose of improving the image. A computed tomography apparatus is disclosed in German OS 35 46 233 wherein a correction of displacements of the rotational center ensues by computation.